1. Field of the Invention
The present invention relates to a communication system using an HSDPA (High Speed Downlink Packet Access) scheme, and more particularly to an apparatus and method for processing an MAC-hs (Medium Access Control-high speed) PDU (Protocol Data Unit) in an HSDPA (High Speed Downlink Packet Access) communication system.
2. Description of the Related Art
Generally, an HSDPA (High Speed Downlink Packet Access) scheme has developed into an HS-DSCH (High Speed-Downlink Shared Channel) for supporting downlink high-speed packet transmission in a W-CDMA communication system, its associated control channels, and devices, systems, and methods for the HS-DSCH. An HARQ (Hybrid Automatic Retransmission Request) scheme has been proposed to support the HSDPA scheme. A detailed configuration and an HARQ scheme for use in the W-CDMA communication system will hereinafter be described with reference to FIG. 1.
FIG. 1 is a block diagram illustrating a conventional W-CDMA communication system. Referring to FIG. 1, the W-CDMA communication system includes a Core Network (CN) 100, a plurality of RNSs (Radio Network Subsystems) 110 and 120, and a UE (User Equipment) 130. The first RNS 110 and the second RNS 120 each includes an RNC (Radio Network Controller) and a plurality of Node Bs. It should be noted that each RNS will hereinafter be referred to as an RNC, and the plurality of Node Bs will be referred to as a cell for the convenience of description. For example, the RNS 110 includes an RNC 111 and a plurality of Node Bs 113 and 115. The RNC is classified into a Serving RNC (SRNC), a Drift RNC (DRNC), and a Controlling RNC (CRNC) according to its various purposes. Particularly, the RNC is classified into the SRNC and the DRNC according to the functions associated with individual UEs. A specific RNC capable of managing the UE's information and transmitting data to the core network is an SRNC of the UE. If the UE's data is transmitted to the SRNC over another RNC instead of the SRNC, the RNC is a DRNC of the UE. The CRNC indicates an RNC for controlling individual Node Bs. For example, if the RNC 111 manages information of the UE 130 as shown in FIG. 1, the RNC 111 is an SRNC. If the UE 130 moves to another position and thus the UE 130's data is transmitted or received using the RNC 112, the RNC 112 is a DRNC. The RNC 111 for controlling the Node B 113 is a CRNC of the Node B 113.
The HARQ scheme, particularly, an n-channel SAW-HARQ (n-channel Stop And Wait Hybrid Automatic Retransmission Request) scheme will hereinafter be described. The conventional ARQ (Automatic Retransmission Request) scheme enables an ACK (Acknowledgement) signal and retransmission packet data to be interchanged between a UE and an RNC (Radio Network Controller). The HARQ scheme generally uses an FEC (Forward Error Correction) method to increase the transmission efficiency of the ARQ scheme. The HSDPA scheme enables the ACK signal and the retransmission packet data to be interchanged between the UE and an MAC HS-DSCH of the RNC. The HSDPA scheme uses N logical channels, and thereby uses the n-channel SAW HARQ method capable of transmitting a plurality of packet data elements even though there is no ACK signal. The SAW ARQ (Stop And Wait Retransmission Request) scheme only transmits the next packet data after receiving the ACK signal associated with previous packet data, resulting in deterioration of channel use efficiency. The n-channel SAW HARQ scheme successively transmits a plurality of packet data units to a destination over a different channel since there is no ACK signal associated with the previous packet data, resulting in an increased channel use efficiency. In more detail, provided that N channels are established between the UE and the Node B, and individual N channels can be distinguished from each other using a specific time or a specific channel number, the UE receiving the packet data can recognize which of the channels is adapted to transmit packet data received at a predetermined time, may reconstruct a plurality of packet data units in the order of data receptions or may combine corresponding packet data units in the order that they were received to perform necessary operations.
Operations of the n-channel SAW HARQ scheme will hereinafter be described with reference to FIG. 1. Firstly, the n-channel SAW HARQ scheme, particularly, the 4-channel SAW HARQ scheme, is performed between the UE 130 and the Node B 113. It is assumed that each channel of the four channels is assigned a logical ID (Identifier). A Medium Access Control (MAC) layer between the UE 130 and the Node B 113 includes HARQ processors corresponding to individual channels. The Node B 113 assigns a channel ID “1” to a coding unit used for the initial data transmission, and transmits the channel ID “1” to the UE 130. In this case, the channel ID is clearly assigned to the coding unit. If an error occurs in the coding unit due to the channel ID “1”, the UE 130 transmits an NACK (Negative Acknowledgement) signal associated with the coding block to the HARQ processor (i.e. HARQ processor 1) corresponding to the channel ID “1” via the Node B 113. In this case, the Node B can transmit subsequent coding blocks over a prescribed channel 2 without taking into consideration the ACK arrival information associated with a coding block of the channel 1. Upon receiving the NACK signal of the channel 1's coding block from the UE 130, the Node B 113 re-transmits a corresponding coding block over the channel 1. Therefore, the UE 130 recognizes that the coding block having been transmitted over the channel 1 is retransmitted using the channel ID of the re-transmitted coding block, and transmits the retransmission coding block to the HARQ processor 1. The HARQ processor 1 receiving the retransmission coding block combines the prestored initial transmission coding block with the retransmission coding block. The n-channel SAW HARQ scheme connects the channel ID to the HARQ processor on a one to one basis, and can properly match the initial transmission data with the retransmission data without delaying the user data transmission execution time by having to wait for the ACK signal.
The hierarchical structure of the W-CDMA system using the HSDPA scheme further requests an HARQ function from the MAC layer, such that a new hierarchical structure corresponding to the requested HARQ function changes from the typical hierarchical structure, i.e. a W-CDMA communication system which does not use the HSDPA scheme. In order to support the HSDPA scheme, the MAC-hs entity is further implemented in individual MAC-c/sh (Medium Access Control-common/shared) and MAC-d (Medium Access Control-common/redrafted) entities in the MAC layer structure for use in the conventional W-CDMA communication system.
FIG. 2 is a diagram illustrating an MAC-hs layer configuration of a UE for use in a CDMA communication system using the HSDPA scheme. Referring to FIG. 2, the MAC-hs sub-layer 115 adapts an HARQ function for use in the HS-DSCH capable of supporting an HSDPA scheme as a basic function. The MAC-hs sub-layer 115 transmits the ACK signal to the Node B when there is no error associated with a data block (i.e. packet data) received from a radio channel. If there is an error associated with the data block received from the radio channel, the MAC-hs sub-layer 115 generates an NACK signal for requesting the retransmission of the, and transmits the NACK signal to the Node B. The MAC-hs sub-layer 115 receives predetermined setup information from the RRC.
The data block transmitted to the MAC-hs sub-layer 115 over the HS-DSCH is stored in one of many HARQ processers contained in the HARQ block. In this case, an HARQ process ID contained in a downlink control signal indicates which of the HARQ processers will store the data block. The HARQ processer having stored the data block transmits NACK information to the UTRAN when an unexpected error occurs in the data block, thus requesting the retransmission of the data block from the UTRAN. If there is no error in the data block, the HARQ processer transmits the data block to a reordering block, and transmits a ACK information to the UTRAN. There is a plurality of reordering blocks for each priority level, which is similar to a transmission buffer of the UTRAN. The HARQ process transmits the data block to a corresponding reordering block through a PCI (Priority Class Identifier) contained in the data block. The most important characteristic of the reordering block is that the reordering block can support a sequential transmission of data. The data block is sequentially transmitted to an upper layer on the basis of a TSN (Transmission Sequence Number). If previous data blocks of the corresponding data block are not received, the corresponding data block is first stored in a buffer, and is then transmitted to the upper layer on the condition that all of the previous data blocks have been received. Typically, the reordering block non-sequentially receives the data blocks because a plurality of HARQ processes are operated at the same time. The reordering buffer is required to sequentially transmit the data blocks to the upper layer. Data blocks of certain TSNs are maintained in the reordering buffer, and at least one data block corresponding to a TSN which is less than the TSNs of the data blocks maintained in the reordering buffer is omitted, such that a stall phenomenon occurs when the data blocks cannot be transmitted to the upper layer.
In the case where there is no CRC error in the MAC-hs PDU that indicates an output signal of an HSDPA MAC-hs decoding chain configured in the form of a hardware device, a corresponding MAC-hs PDU is transmitted to the upper layer MAC layer at predetermined time intervals equal to an integer multiple of the HS-DSCH TTI (2 ms). Based on the 3GPP (3rd Generation Partnership Project) Release 5 TS 25.306 specification, the higher the HS-PDSCH category level, the shorter the transmission interval. The MAC-hs PDU received at the predetermined time intervals denoted by “Inter-minimum TTI interval (=N)×HS-DSCH TTI (2 ms)” is transmitted to the upper MAC layer on the basis of a new definition called “Inter-minimum TTI interval”. The upper layer (MAC) must read output data of the HSDPA MAC-hs layer at the predetermined time intervals denoted by “N×HS-DSCH TTI”, for example, at intervals of 2 ms. Provided that the hardware does not carry out the buffering operation, the MAC layer typically operating in a controller must unavoidably read the MAC-hs PDU value at maximum intervals of 2 ms. However, it may be difficult for a typical OS (Operating System) widely used for the UE to perform an interrupt handling operation adapted to execute a predetermined data processing function at intervals of 2 ms.